In a highly complex modern society, a reduction of willingness turns into a problem. For example, the term “motivation crisis” is used to describe the problem of reduced motivation in young people. Moreover, it is said that depression patients often show symptoms of hypobulia, and there is a demand for the development of drugs capable of improving hypobulia.
In recent years, a “dipeptide” composed of two amino acids attached to each other has received attention as a functional substance. Dipeptides can be designed to have additional physical properties and/or new functions, which cannot be seen in single amino acids, and hence they are expected to be applicable to a wider range than that of amino acids. The inventors of the present invention have found that a 2,5-diketopiperazine derivative, i.e., 2,5-piperazinedione,3,6-bis(phenylmethyl)-,(3S,6S)- (CA Registry Number: 2862-51-3) (hereinafter referred to as “compound A”) has an improving effect on learning motivation (the co-filed patent application: Japanese Patent Application No. 2009-296164).
On the other hand, most consumers prefer acidic beverages. Products known to contain compound A include chicken extracts in liquid form which are obtained by decocting meat of livestock, as well as chicken consomme and the like in solid form.